The curly braces are the syntax for lambdas, one of the basic types `(T) -> R?`. There are plenty of examples for using that, but in the respective chapters: https://kotlinlang.org/docs/reference/lambdas.html#higher-or... or in this bunch of examples: https://kotlinlang.org/docs/reference/idioms.html#filtering-...

The docs are arranged in a way that one reads them in the order they are written (at least the first half). I supposed that would take more than half an hour given at the competition.

I don't think Kotlin is the best choice for programming challenges like codeforces - they really don't let him shine (although I use it whenever I can). But if this is a valid case, we can collaborate and write a short tutorial for C/C++ to Kotlin programmers.